Lithography is arguably the most critical and costly set of operations in fabricating integrated circuits. Lithography is a process used to transfer a pattern from a mask or reticle to a layer of resist deposited on the surface of a wafer. A mask or reticle may be a square glass or quartz plate with a patterned metal film such as chrome on one side. Further, different variations of lithography utilize different wavelengths of radiation to expose the photoresist. In particular, photolithography (or optical lithography) may use UV radiation, X-ray lithography may use X-rays, e-beam lithography may use an electron beam, and ion beam lithography may use an ion beam. The exposure transfers the pattern from the mask to the wafer's surface by physically altering the photoresist.
Masks account for a large percentage of the cost of lithography. One semiconductor wafer may include many layers of circuitry, and a different mask may be used for each layer. For leading edge technologies, a mask set may cost $750,000. And, a large percentage of that money is spent controlling line widths. In particular, the industry drive toward smaller components has forced masks to include tiny, intricate details In fact, the dimensions of the circuit patterns on the masks are so small that they are close to the wavelength of light used to expose the wafer's surface. Further, because the dimensions of the circuit patterns and the wavelength of the light, a straight line printed on (or outlined by) the mask may not necessarily expose the wafer's surface to the straight line. The wafer's surface may be exposed to a line that is curved and rounded at the corners, creating a pattern in the photoresist that is slightly off and potentially causing a short or other defect in the resulting semiconductor circuit. Thus, an increasing number of error corrective techniques including, e.g., optical interference effects and diffraction effects, have been formulated to avoid such problems.
After such a large investment in capital, time, and effort to create masks, completed masks may still be discarded because a portion of the circuit pattern needs to be updated or changed after manufacturing the mask. To combat the costs of changing circuit patterns after a mask set is produced, various solutions have been employed to repair masks. However, the cost and cycle time associated with making repairs to a mask set are substantial and there are additional considerations to address with mask repair. For instance, a short in a circuit pattern created on a wafer may be discovered during testing of a first run in production of semiconductor circuits and, if the affected circuitry cannot be bypassed without impairing the functionality of the semiconductor circuit, the mask(s) may be removed from the production line for repair and the entire production line may be stopped until the mask(s) are repaired.
One solution is focused-ion beam (FIB) repair, which involves focusing a tight beam of ions on the mask to mill material away from a surface or deposit material on the surface. For example, to connect two unconnected lines, an FIB can deposit a small amount of aluminum in a small stripe between the two lines on a wafer. Or, in another example, FIB may repair a mask by milling away a portion of chrome contamination off the surface of a mask. In yet another example, FIB may deposit a little bar of chrome on the mask.
Repairing the mask prevents repetition of the defect in all the semiconductor circuits and saves the cost of making an entirely new mask. However, FIB mask repair is limited to minor changes to a mask and is a “one-time only” technique. FIB requires that the mask be removed from the lithography equipment, inserted into the FIB system, repaired, cleaned, and replaced back into the lithography equipment. As a result, repairs are very time-consuming and additional repairs may significantly deteriorate the integrity of the circuit pattern on the mask.
Beyond the capital, time, and effort expended for reparations of masks that result from problems with a mask set, small differences between product lines of semiconductor circuits such as changes to a clock circuit to modify the frequency of operations in the semiconductor circuit may require the creation of additional mask sets. Some design decisions are preferably made at the time of manufacture of semiconductor circuits. For example, design decisions may include a decision about clock frequency for the circuit because design decisions for the clock frequency may be based upon the market to which the product will be sold and current market conditions.
Therefore, there is a need for methods and arrangements capable of real-time changes in a circuit design of a mask.